Many motors can be wired for alternate voltages. Your motor may be wired to a voltage different than what your static converter is supplying.
It certainty is possible. I checked the serial on the mill, it's a 1963. the static phase converter that the seller sold me with the mill is rated for min 4 hp to max 8hp. seems like a lot of hp for a Bridgeport motor of this age. But I really don't know anything about the motor. it's going to a motor shop tomorrow and I'll be ordering a VFD to replace the phase converter.